Since the enthalpy of elements in their natural state is zero, the heat of formation Hf of compounds:
Correct answer: D. Is zero.
- A. Is always negative.
- B. Is always positive.
- C. May be negative or positive.
- D. Is zero.
Explanation
The enthalpy of formation, denoted as Hf, is defined as the change in enthalpy when one mole of a compound is formed from its elements in their standard states. For elements in their natural state, the standard enthalpy of formation is zero because no formation from other elements is involved. Option D is correct because it aligns with this definition. Options A, B, and C are incorrect because they do not accurately reflect the enthalpy of formation for elements in their natural state.
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About Thermochemistry and Energetics of Chemical Reactions
Thermochemistry measures energy changes in chemical reactions and distinguishes exothermic from endothermic processes. Work covers systems, surroundings and state functions, internal energy, the first law of thermodynamics, enthalpy and Hess's law, including the sign conventions used when heat enters or leaves a system.
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